Method and device for arithmetic coding of video, and method and device for arithmetic decoding of video

ABSTRACT

Methods and apparatuses for arithmetic encoding/decoding of video data. The arithmetic decoding method includes arithmetically decoding prefix bit strings representing a two-dimensional location of a last significant coefficient in a block sequentially by using a context model, arithmetically decoding suffix bit strings in a bypass mode, and performing inverse binarization on the arithmetically decoded prefix bit strings and suffix bit strings to acquire the location of the last significant coefficient in the block.

TECHNICAL FIELD

The present invention relates to encoding and decoding of video, andmore particularly, to a method and apparatus for entropy encoding anddecoding transformation coefficient information.

BACKGROUND ART

In an image compression method such as moving picture experts group(MPEG)-1, MPEG-2, and MPEG-4 H.264/MPEG-4 AVC (advanced video coding),an image is divided into blocks, each having a predetermined size, andresidual data of each block is acquired by using an inter prediction oran intra prediction process. The residual data is compressed throughprocesses such as transformation, quantization, scanning, run lengthcoding, and entropy coding. During the entropy coding operation, asyntax element, for example, information about, for example, atransformation coefficient or a prediction mode, is entropy encoded, anda bit stream is output. A decoder parses the bit stream to extractsyntax elements, and restores image based on the extracted syntaxelements.

DISCLOSURE Technical Problem

The present invention provides a method and apparatus for arithmeticallyencoding and decoding symbols of transformation coefficients bycombining a context adaptive binary arithmetic coding (CABAC) methodthat has high calculation complexity while having a high compressiveperformance, and a bypass arithmetic coding method having lowercalculation complexity than that of the CABAC method while showing lesscompressive throughput.

Technical Solution

According to the present invention, symbols regarding a location of alast significant coefficient is classified into a prefix and a suffixbased on a predetermined critical value, and bit string classified asthe prefix and bit string classified as the suffix are independentlyarithmetic encoded.

Advantageous Effects

According to the embodiments of the present invention, processing speedand compressive performance of arithmetic encoding and decoding can beimproved through trade-off between calculation complexity and processingspeed, by applying the CABAC method that has high calculation complexitywhile having a high compressive performance, and the bypass arithmeticcoding method having lower calculation complexity than that of the CABACmethod while showing less compressive throughput.

DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram of a video encoding apparatus according to anembodiment of the present invention;

FIG. 2 is a block diagram of a syntax element coding unit shown in FIG.1;

FIG. 3 is a flowchart illustrating processes of arithmetic encoding anddecoding transformation coefficient information in each block, accordingto an embodiment of the present invention;

FIG. 4 is a diagram showing sub-sets obtained by dividing a block inorder to perform arithmetic encoding on a syntax element relating to atransformation coefficient, according to an embodiment of the presentinvention;

FIG. 5 is a reference diagram illustrating a process of arithmeticencoding a location of a last significant coefficient by classifying thelocation into a prefix bit string and a suffix bit string according toan embodiment of the present invention;

FIG. 6 is a diagram illustrating a process of arithmetically encodinglocation information of the last significant coefficient according to anembodiment of the present invention;

FIG. 7 is a diagram illustrating an arithmetic encoding process using acontext model performed on a regular coding unit shown in FIG. 2;

FIG. 8 is a flowchart illustrating an arithmetic encoding method forencoding video, according to an embodiment of the present invention;

FIG. 9A is a block diagram showing a video encoding apparatus accordingto an embodiment of the present invention;

FIG. 9B is a block diagram showing an arithmetic decoding apparatusincluded in a syntax element decoding unit of FIG. 9A;

FIG. 10 is a flowchart illustrating an arithmetic decoding method fordecoding video, according to an embodiment of the present invention;

FIG. 11 is a block diagram of a video encoding apparatus according to anembodiment of the present invention;

FIG. 12 is a block diagram of a video decoding apparatus according to anembodiment of the present invention;

FIG. 13 is a diagram showing a concept of a coding unit according to anembodiment of the present invention;

FIG. 14 is a block diagram of a video encoding apparatus based onhierarchical coding units according to an embodiment of the presentinvention;

FIG. 15 is a block diagram of a video decoding apparatus based onhierarchical coding units according to an embodiment of the presentinvention;

FIG. 16 is a diagram illustrating coding units according to depths, andpartitions, according to an embodiment of the present invention;

FIG. 17 is a diagram for describing a relationship between a coding unitand transformation units, according to an embodiment of the presentinvention;

FIG. 18 is a diagram for describing encoding information of coding unitscorresponding to a coded depth, according to an embodiment of thepresent invention;

FIG. 19 is a diagram of coding units according to depths, according toan embodiment of the present invention;

FIGS. 20 through 22 are diagrams for describing a relationship betweencoding units, prediction units, and frequency transformation units,according to an embodiment of the present invention; and

FIG. 23 is a diagram for describing a relationship between a codingunit, a prediction unit, and a transformation unit, according toencoding mode information of Table 1.

BEST MODE

According to an aspect of the present invention, there is provided anarithmetic decoding method for decoding video, the method comprising:acquiring a first coordinate prefix syntax element and a firstcoordinate suffix syntax element respectively corresponding to a firstcoordinate prefix bit string and a first coordinate suffix bit stringthat are obtained by classifying a first coordinate location of a lastsignificant coefficient of a current block based on a critical valuedetermined based on a size of the current block, and a second coordinateprefix syntax element and a second coordinate suffix syntax elementrespectively corresponding to a second coordinate prefix bit string anda second coordinate suffix bit string that are obtained by classifying asecond coordinate location of the last significant coefficient based onthe critical value, from a received bit stream; acquiring the firstcoordinate prefix bit string and the second coordinate prefix bit stringby performing arithmetic decoding operations sequentially with respectto the first coordinate prefix syntax element and the second coordinateprefix syntax element in a first arithmetic decoding method; acquiringthe first coordinate suffix bit string and the second coordinate suffixbit string by performing arithmetic decoding operations sequentiallywith respect to the first coordinate suffix syntax element and thesecond coordinate suffix syntax element in a second arithmetic decodingmethod; and restoring the first coordinate component by performinginverse binarization on the first coordinate prefix bit string and thefirst coordinate suffix bit string, and restoring the second coordinatecomponent by performing inverse binarization on the second coordinateprefix bit string and the second coordinate suffix bit string.

According to another aspect of the present invention, there is provideda video decoding apparatus comprising: a parser acquiring a firstcoordinate prefix syntax element and a first coordinate suffix syntaxelement respectively corresponding to a first coordinate prefix bitstring and a first coordinate suffix bit string that are obtained byclassifying a first coordinate location of a last significantcoefficient of a current block based on a critical value determinedbased on a size of the current block, and a second coordinate prefixsyntax element and a second coordinate suffix syntax elementrespectively corresponding to a second coordinate prefix bit string anda second coordinate suffix bit string that are obtained by classifying asecond coordinate location of the last significant coefficient based onthe critical value, from a received bit stream; and an arithmeticdecoder acquiring the first coordinate prefix bit string and the secondcoordinate prefix bit string by performing arithmetic decodingoperations sequentially with respect to the first coordinate prefixsyntax element and the second coordinate prefix syntax element in afirst arithmetic decoding method, acquiring the first coordinate suffixbit string and the second coordinate suffix bit string by performingarithmetic decoding operations sequentially with respect to the firstcoordinate suffix syntax element and the second coordinate suffix syntaxelement in a second arithmetic decoding method, and restoring the firstcoordinate component by performing inverse binarization on the firstcoordinate prefix bit string and the first coordinate suffix bit string,and restoring the second coordinate component by performing inversebinarization on the second coordinate prefix bit string and the secondcoordinate suffix bit string.

According to another aspect of the present invention, there is providedan arithmetic encoding method for encoding a video, the methodcomprising: classifying a first coordinate component of a location of alast significant coefficient in a current block into a first coordinateprefix bit string and a first coordinate suffix bit string based on acritical value that is determined based on a size of the current block;classifying a second coordinate component of the location of the lastsignificant coefficient into a second coordinate prefix bit string and asecond coordinate suffix bit string based on the critical value;performing arithmetic encoding on the first coordinate prefix bit stringand the second coordinate prefix bit string sequentially in a firstarithmetic encoding method; and performing arithmetic encoding on thefirst coordinate suffix bit string and the second coordinate suffix bitstring in a second arithmetic encoding method.

According to another aspect of the present invention, there is provideda video encoding apparatus comprising: an image encoder performingprediction, transformation, and quantization on blocks that are obtainedby splitting video for generating syntax elements of the blocks; and anentropy encoder classifying a first coordinate component of a locationof a last significant coefficient in a current block into a firstcoordinate prefix bit string and a first coordinate suffix bit stringbased on a critical value that is determined based on a size of thecurrent block, classifying a second coordinate component of the locationof the last significant coefficient into a second coordinate prefix bitstring and a second coordinate suffix bit string based on the criticalvalue, performing arithmetic encoding on the first coordinate prefix bitstring and the second coordinate prefix bit string sequentially in afirst arithmetic encoding method, and performing arithmetic encoding onthe first coordinate suffix bit string and the second coordinate suffixbit string in a second arithmetic encoding method.

MODE FOR INVENTION

Hereinafter, embodiments of the present invention will be described indetail with reference to accompanying drawings. In the followingdescription, the term ‘image’ may refer to a still image or a movingpicture, that is, a video.

First, an arithmetic encoding method and a video encoding apparatusaccording to an embodiment of the present invention, and an arithmeticdecoding method and a video decoding apparatus according to anembodiment of the present invention will be described with reference toFIGS. 1 through 10.

FIG. 1 is a block diagram of a video encoding apparatus 10 according toan embodiment of the present invention.

The video encoding apparatus 10 splits a picture from among a pluralityof pictures configuring the video into data units of a hierarchicalstructure, and performs prediction, transformation, and quantization byusing the data units of the hierarchical structure. As will be describedlater with reference to FIGS. 11 through 23, the data units of thehierarchical structure may be maximum coding units, coding units,prediction units, or transformation units. The prediction units and thetransformation units used in a prediction process and a transformationprocess may be determined independently from other data units based oncosts.

Since encoding is recursively performed on each of coding units having ahierarchical structure in each region of a maximum coding unit todetermine an optimum coding unit, data units having a tree structure maybe obtained. That is, the coding units having a tree structure, andprediction units and transformation units having a tree structure may bedetermined in each maximum coding unit. Hierarchical informationrepresenting configurations of the data units having the hierarchicalstructure and non-hierarchical information for decoding have to betransmitted to perform a decoding operation.

Information relating to the hierarchical structure is necessary todetermine coding units having a tree structure, prediction units havinga tree structure, and transformation units having a tree structure,which will be described below, and may include a size of the maximumcoding unit, a coded depth, partition information of the predictionunits, a slit flat representing whether the coding units are split, sizeinformation of the transformation units, and a transformation unit (TU)size flag representing whether the transformation units are split.Encoding information other than the hierarchical structure informationmay include prediction mode information of intra/inter predictionapplied to each of the prediction units, motion vector information,prediction direction information, color component information applied tocorresponding data units when a plurality of color components are used,and transformation coefficient information. Hereinafter, thehierarchical information and non-hierarchical information that are to beentropy encoded and decoded may be referred to as a syntax element or asymbol. In addition, for convenience of description, data units arereferred to as blocks. The blocks correspond to prediction units orpartitions during the prediction process, and correspond totransformation units during the transformation process.

Referring to FIG. 1, the video encoding apparatus 10 includes an imageencoder 11 and a syntax element encoder 12.

The image encoder 11 performs operations such as prediction,transformation, and quantization of image blocks to generate syntaxelements. In more detail, the image encoder 11 generates a predictionvalue through an intra prediction and an inter prediction in each ofblocks, and generates a transformation coefficient by transforming andquantizing a residual that is a difference between an original block andthe prediction value.

The syntax element encoder 12 performs arithmetic encoding of syntaxelements regarding the transformation coefficient generated in each ofthe blocks and other various syntax elements generated in other encodingprocesses, to generate a bit stream. In particular, the syntax elementencoder 12 of the present embodiment classifies a row location and acolumn location of a last significant coefficient that is scanned lastaccording to a predetermined scanning order among the significantcoefficients except for 0 in the block into prefixes and suffixes basedon a critical value determined according to a width or a height of thecurrent block, and arithmetically encodes bit strings that are obtainedby performing binarization on the prefixes sequentially by applying acontext model according to context adaptive binary arithmetic coding(CABAC) and arithmetically encodes bit strings that are obtained byperforming binarization on the suffixes in a bypass mode after the CABACis performed.

FIG. 2 is a block diagram showing the syntax element encoder 12 of FIG.1 in more detail.

Referring to FIG. 2, a syntax element encoder 20 includes a binarizer21, a context modeller 22, and a binary arithmetic coder 23. Also, thebinary arithmetic coder 23 includes a regular coding engine 24 and abypass coding engine 25.

If the syntax elements have not binary values, the binarizer 21binarizes the syntax elements to output a bit string consisting ofbinary values of 0 or 1, that is, a bin string. A bin denotes each bitin the bit string consisting of 0 or 1. According to a type of thesyntax element, one of unary binarization, truncated unary binarization,exponential Golomb binarization, and fixed length binarization may beused.

In particular, the binarizer 21 of the embodiment of the presentinvention classifies the currently encoded syntax element into a prefixor a suffix based on a critical value that is determined according to asize of the current block, and may binarize the prefix and the suffix byusing binarization methods that are independent from each other tooutput a prefix bit string and a suffix bit string. For example, thebinarizer 21 classifies a column location of the last significantcoefficient into the prefix and the suffix based on a critical valuethat is determined based on a width of the current block, and then,outputs a prefix bit string of the column location by applying apredetermined first binarization method to the prefix of the columnlocation and outputs a suffix bit string of the column location byapplying a predetermined second binarization method to the suffix of thecolumn location. The first binarization method and the secondbinarization method are independent from each other, that is, these maybe the same as or different from each other.

Similarly, the binarizer 21 may classify a row location of the lastsignificant coefficient into the prefix and the suffix based on acritical value that is determined based on a height of the currentblock, and then, output a prefix bit string of the row location byapplying a predetermined first binarization method to the prefix of therow location and output a suffix bit string of the row location byapplying a predetermined second binarization method to the suffix of therow location. A process of classifying the currently encoded syntaxelement into the prefix and the suffix will be described below withreference to FIG. 5.

According to the type of syntax element, each of the bins in the bitstring may be arithmetically coded in the regular coding engine 24 byusing the context model, or may be arithmetically coded in the bypasscoding engine 25. In particular, when the location of the lastsignificant coefficient is classified into the prefix bit string and thesuffix bit string, the regular coding engine 24 of the presentembodiment arithmetically encodes the bit strings classified as theprefixes by applying the context model according to the CABACsequentially, and the bypass coding engine 25 may arithmetically encodethe bit strings classified as the suffixes in the bypass mode.

The context modeler 22 provides the regular coding engine 24 with thecontext model for arithmetically encoding the current syntax element. Inparticular, the context modeler 22 outputs a probability of generating abinary value for encoding each of the bins in the prefix bit string tothe regular coding engine 24, when the regular coding engine 24arithmetically encodes the prefix bit string of the last significantcoefficient location. The context model is a probability model of eachbin, and includes information about which one of 0 and 1 corresponds toa most probable symbol (MPS) and a least probable symbol (LPS), andprobability information of the MPS or LPS. The context modeler 22updates the context model according to whether the value of the bitencoded in the regular coding engine 24 is 0 or 1.

The regular coding engine 24 performs arithmetic coding on each of binsconfiguring the prefix bit string of the last significant coefficientlocation based on the context model provided from the context modeler22, that is, information about the MPS and the LPS and probabilityinformation of the MPS or LPS.

The bypass coding engine 25 performs the arithmetic coding of the suffixbit string of the last significant coefficient location according to thebypass mode. In the bypass mode, a probability of generating binarysignals of 0 and 1 has a fixed value. Therefore, the bypass codingengine 25 may directly perform the arithmetic coding on the input bitstring, unlike the arithmetic coding method using the context model, inwhich the context model is updated while arithmetically encoding each ofthe bins and the updated context model is used in the arithmetic codingof the next bin.

FIG. 7 is a diagram illustrating the arithmetic coding processes usingthe context model performed in the regular coding engine 24 of FIG. 2.In FIG. 7, a binary value of the currently encoded symbol is “010”, andit is assumed, for convenience of description, that the probability ofgenerating 1 has a fixed value of 0.2 and the probability of generating0 has a fixed value of 0.8. In actuality, the regular coding engine 24updates the probability of generating the binary value according to thearithmetic coding of each of the bins in the binary value “010”.

Referring to FIG. 7, when a first bin value “0” in the binary value of“010” is encoded, a lower 80% portion, that is, [0.0˜0.8], in an initialsection [0.0˜1.0] is updated as a new section. In addition, when a nextbin value “1” is encoded, an upper 20% portion, that is, [0.64˜0.8], inthe section [0.0˜0.8] is updated as a new section. In addition, when anext bin value “0” is encoded, a lower 80% portion, that is,[0.64˜0.768], in the section [0.64˜0.8] is updated as a new section. Ina binary number 0.11 corresponding to a real number 0.75 included in thefinal section [0.64˜0.768], the number “11” below the decimal pointexcept for the initial number 0 is output as a bit string correspondingto the binary value “010” of the encoded symbol.

Hereinafter, the arithmetic encoding and decoding processes of theinformation relating to the transformation coefficient, that is, syntaxelements relating to the transformation coefficient, will be describedin detail below.

FIG. 3 is a flowchart illustrating processes of arithmetically encodingand decoding transformation coefficient information in each block,according to an embodiment of the present invention.

Referring to FIG. 3, in operation 31, a flag coded_block_flagrepresenting whether there is a significant coefficient other than 0among the transformation coefficients included in the current block isarithmetically encoded or decoded first.

If the current block only has transformation coefficients that are 0 anddoes not have a significant coefficient other than 0, a value 0 isarithmetically encoded as the coded_block_flag, and arithmetic encodingprocesses of the syntax elements relating to the other transformationcoefficients are omitted.

In operation 32, if there is a significant coefficient in the currentblock, that is, a value of coded_block_flag is 1, a significance map(SigMap) indicating a location of the significant coefficient isarithmetically encoded or decoded.

The significance map SigMap may consist of predetermined informationindicating a significant bit and a location of the last significantcoefficient. The significant bit represents whether the transformationcoefficient according to each scan index is the significant coefficientor 0, and may be expressed by using significant_coeff_flag[i]. Thesignificance map SigMap may be set with respect to each of sub-setshaving a predetermined size, which are obtained by splitting the currentblock. That is, significant_coeff_flag[i] may indicate whether thetransformation coefficient of an i-th scan index among thetransformation coefficients included in one sub-set of the current blockis 0 or not.

In the conventional H.264 standard, a flag End-Of-Block that indicateswhether the significant coefficient is the last significant coefficientis separately arithmetic encoded or decoded in each of the significantcoefficients. However, according to the present embodiment, theinformation about the location of the last significant coefficient isarithmetically encoded or decoded as itself. For example, if the lastsignificant coefficient is located at an x-th column (where x is aninteger) and a y-th row (where y is an integer) of the current block,that is, if the location of the last significant coefficient is (x,y),values of x and y are arithmetically encoded or decoded.

In particular, the syntax element encoder 12 according to the presentembodiment classifies a row location (x) of the last significantcoefficient into an x-axis prefix bit string (or a first coordinateprefix bit string) and an x-axis suffix bit string (or a firstcoordinate suffix bit string) based on a critical value that isdetermined based on a width of the current block, and classifies acolumn location (y) of the last significant coefficient into an y-axisprefix bit string (or a second coordinate prefix bit string) and any-axis suffix bit string (or a second coordinate suffix bit string)based on a critical value that is determined based on a height of thecurrent block. In addition, the syntax element encoder 12 performs thearithmetic encoding of the x-axis and y-axis prefix bit strings byapplying the context model according to the CABAC, and performs thearithmetic encoding on the x-axis and y-axis suffix bit strings in thebypass mode. Also, the syntax element encoder 12 may perform thearithmetic encoding sequentially by forming groups of the prefix bitstrings and the suffix bit strings that are respectively encoded usingdifferent arithmetic encoding types. That is, the syntax element encoder12 performs the arithmetic encoding on the x-axis prefix bit string andthe y-axis prefix bit string by applying the context model, and mayperform the arithmetic encoding on the x-axis suffix bit string and they-axis suffix bit string in the bypass mode, after the arithmeticencoding performed on the prefix bit strings or independently from thearithmetic encoding process performed on the prefix bit strings.

FIG. 4 is a diagram showing sub-sets obtained by splitting the block inorder to arithmetically encode syntax elements relating to thetransformation coefficients. In FIG. 4, reference numeral 41 denotes thelast significant coefficient in the block 40.

Referring to FIG. 4, in order to perform the arithmetic coding of thesyntax elements relating to the transformation coefficients included inthe block 40, the block 40 may be split into sub-sets havingpredetermined sizes. The transformation coefficient information mayinclude a significance map (SigMap), a flag indicating whether thesignificant coefficient included in the sub-set is greater than 1(Greaterthan1 flag), a flag indicating whether the significantcoefficient included in the sub-set is greater than 2 (Greaterthan2flag), information representing a level of the significant coefficientsgreater than 2 (level-3), and information about a location of the lastsignificant coefficient 41. The syntax elements such as SigMap,Greatherthan1 flag, Greatherthan2 flag, and (level-3) may bearithmetically encoded in the sub-set unit. A processing order of thesub-sets may be variously set. For example, as denoted by arrows, thetransformation coefficient information included in each of the sub-setsmay be arithmetically encoded in an order from subset15 to subset0. Thepresent invention is not limited thereto, and if a size of the block 40is small, the block may not be split into the sub-sets, but syntaxelements such as SigMap, Greaterthan1 flag, Greaterthan2 flag, andlevel-3 included in the block 40 may be arithmetically encoded.

As described above, among the significant coefficients included in theblock 40, a row location (x) and a column location (y) of the lastsignificant coefficient 41 according to a predetermined scanning orderare respectively classified into a prefix bit string and a suffix bitstring based on the size of the current block 40. Then, the prefix bitstrings are arithmetically encoded by applying the context modelaccording to the CABAC and the suffix bit strings are arithmeticallyencoded in the bypass mode.

Hereinafter, processes of classifying the location of the lastsignificant coefficient into the prefix bit string and the suffix bitstring will be described in more detail below.

FIG. 5 is a referential diagram illustrating processes of classifyingthe location of the last significant coefficient into the prefix bitstring and the suffix bit string, and arithmetically encoding the bitstrings.

When it is assumed that a width of the block is w (where w is aninteger) and a height of the block is h (where h is an integer), alocation of the transformation coefficient in the block has atwo-dimensional coordinate value (x, y). When it is defined that alocation of the transformation coefficient is at a left uppermost cornerof the block is (0,0) and a location of the transformation coefficientis at a right lowermost corner of the block ((w−1), (h−1)), the rowlocation x of the transformation coefficient in the block has one ofvalues from 0 to (w−1) and the column location y of the transformationcoefficient has one of values from 0 to (h−1).

First, processes of classifying the row location x into the prefix bitstring and the suffix bit string and arithmetically encoding the bitstrings will be described below.

Referring to FIG. 5, the row location x of the last significantcoefficient in the block may have a value from among 0 to (w−1). Basedon a critical value th determined based on the width w of the block, therow location x is classified into a prefix that is equal to or less thanthe critical value th and a suffix (x-th) exceeding the critical valueth.

In particular, if the value of the row location x of the lastsignificant coefficient is equal to or less than the critical value th,the row location is only classified as the prefix, and not classified asthe suffix. If the value of the row location x exceeds the criticalvalue th, the row location x is classified into the prefix correspondingto the critical value th and the suffix (x-th) exceeding the criticalvalue th. That is, if the row location x has a value that is equal to orless than the critical value th, the row location is only classified asthe prefix and there is no suffix. In addition, only when the rowlocation x exceeds the critical value th, is the row location xclassified into the prefix and the suffix.

In other words, if the row location x of the last significantcoefficient is equal to or less than the critical value th, the rowlocation x is binarized according to a predetermined first binarizationmethod and classified only as the x-axis prefix bit string. If the rowlocation x of the last significant coefficient x exceeds the criticalvalue th, the row location x is classified into the x-axis prefix bitstring that is obtained by performing binarization on the critical valueth in the first binarization method and the x-axis suffix bit stringthat is obtained by performing binarization on the value (x-th) in apredetermined second binarization method.

The critical value th, based on which the row location x is classifiedinto the prefix and the suffix, may be determined based on the width wof the block, as described above. For example, the critical value th maybe (w/2)−1 that is an intermediate value between 0 to (w−1) that is anallowable range of the row location x. As another example, if the widthw of the block is a power of 2, the critical value th may be determinedby the following equation th=(log₂w<<1)−1. The critical value th is notlimited thereto, and may be set in various ways.

In particular, if the width w of the block has a value of 8, thecritical value th is (8/2)−1=3, and thus, the row location x may beclassified into the prefix and the suffix based on the value of 3. Ifthe row location x exceeds 3, for example, the row location x has avalue of 5, since x=th+2=3+2, the row location x is classified into theprefix having a value of 3 and the suffix having a value of 2. If therow location x has a value of 3 or less, the row location x is directlybinarized and classified as the prefix, and there is no suffix.

In the previously described example, the prefix 3 and the suffix 2 arerespectively binarized using different binarization methods. Forexample, the prefix may be binarized through a truncated unarybinarization process, and the suffix may be binarized through a fixedlength binarization process. For convenience of description, in FIG. 5,in a case where the row location x is 5 and the critical value th is 3,the value of 3 that is classified as the prefix is binarized into 0001(52) through unary binarization, and the value of 2 that is classifiedas the suffix is binarized into 010 (54) through a general binarizationprocess.

As described above, the prefix bit string 0001 (52) is arithmeticallyencoded by applying the context model. The context index is determinedwith respect to each of the bins in ‘0001’, and the context model forarithmetically encoding each bin may be determined based on thedetermined context index. The suffix bit string 010 (54) isarithmetically encoded in the bypass mode without performing the processof determining the context modelling. In the bypass mode, since each ofthe bins has the same probability, that is, the value of 0 and the valueof 1 have the same fixed probability value of ½, the input bit string101 (54) may be directly arithmetically encoded without using thecontext model.

As described above, if the row location x has the value that is equal toor less than the critical value th, the row location x is onlyclassified as the prefix, and there is no suffix. In the above describedexample, it is assumed that the row location x has a value of 1 that isless than the critical value th of 3. In this case, the value of the rowlocation x, that is, 1, is only classified into the prefix bit stringthrough a predetermined binarization method, and there is no suffix bitstring. As described above, the prefix bit strings are arithmeticallyencoded by applying the context model, and the arithmetic encodingprocess in the bypass mode is omitted because there is no suffix bitstring.

Similarly to the arithmetic encoding performed after classifying the rowlocation x into the prefix bit string and the suffix bit string, thecolumn location y may be classified into the prefix bit string and thesuffix bit string and may be arithmetically encoded. That is, based on acritical value th determined based on the height h of the block, thecolumn location y is classified into the prefix that is equal to or lessthan the critical value th and the suffix having a value of (y-th) thatexceeds the critical value th. If the column location y has a value thatis equal to or less than the critical value th, the column location isonly classified into the prefix, and there is no suffix. If the columnlocation y is classified into the prefix and the suffix, each of theprefix and the suffix is binarized using an independent binarizationmethod, thereby obtaining a prefix bit string and a suffix bit string.

That is, the column location y of the last significant coefficient isequal to or less than the critical value th, the column location y isbinarized using a first binarization method and classified into they-axis prefix bit string. If the column location y of the lastsignificant coefficient exceeds the critical value th, the columnlocation y is classified into the y-axis prefix bit string that isobtained by performing binarization on the critical value th in thefirst binarization method and a y-axis suffix bit string that isobtained by performing binarization on the value of (y-th) in a secondbinarization method.

In addition, the prefix bit string is arithmetically encoded by applyingthe context model, and the suffix bit string is arithmetically encodedin the bypass mode.

FIG. 6 is a diagram illustrating a process of arithmetically encodingthe location information of the last significant coefficient, accordingto an embodiment of the present invention.

As described above, when the row location x and the column location y ofthe last significant coefficient are respectively classified into theprefix bit strings and the suffix bit strings based on the criticalvalue th that is determined based on the size of the current block, thesyntax element encoder 12 according to the present embodiment performsthe arithmetic coding of the prefix bit strings by applying the contextmodel according to the CABAC and performs the arithmetic coding of thebit strings classified as the suffix in the bypass mode.

Referring to FIG. 6, when the row location x 61 of the last significantcoefficient is classified into an x-axis component prefix 62 and anx-axis component suffix 63 and the column location y is classified intoa y-axis component prefix 66 and a y-axis component suffix 67, thesyntax element encoder 12 makes a group of the x-axis component prefix62 and the y-axis component prefix 66 and performs the arithmeticencoding on the group by applying the context modelling 68. In addition,after performing the context modelling 68 or independently from thecontext modelling 68, the syntax element encoder 12 makes a group of thex-axis component suffix 63 and the y-axis component suffix 67 andperforms the arithmetic encoding on the group in the bypass mode 69. Asdescribed above, the syntax element encoder 12 of the present embodimentmay perform the process with respect to groups of the bit strings towhich the same arithmetic encoding method is applied, and thus,calculation speed may be improved. In particular, the x-axis componentsuffix 63 and the y-axis component suffix 67 are grouped and thearithmetic encoding of the group is performed in the bypass mode, andthus, the suffix bit strings may be arithmetically encoded instantly.Since the bypass mode uses the fixed probability value as describedabove, encoding results corresponding to the entire input bit stringsmay be directly output. In addition, as described above, when the rowlocation x or the column location y of the last significant coefficienthas a value that is equal to or less than the predetermined criticalvalue, the suffix bit strings of the row and column locations do notexist, and thus, the arithmetic encoding process of the suffix bitstrings may be omitted.

FIG. 8 is a flowchart illustrating an arithmetic encoding method forencoding video, according to an embodiment of the present invention.

Referring to FIG. 8, in operation 82, the syntax element encoder 12classifies a first coordinate component of the last significantcoefficient location in the current block into a first coordinate prefixbit string and a first coordinate suffix bit string based on a criticalvalue that is determined based on a size of the current block. Asdescribed above, the syntax element encoder 12 binarizes the rowlocation x according to a first predetermined binarization method andclassifies a result of the the binarization as the x-axis prefix bitstring when the row location x of the last significant coefficient isequal to or less than the critical value th. Also, when the row locationx of the last significant coefficient exceeds the critical value th, thesyntax element encoder 12 classifies the row location x into the x-axisprefix bit string that is obtained by performing binarization on thecritical value th according to the first binarization method and thex-axis suffix bit string that is obtained by performing binarization ona value of (x-th) according to a second predetermined binarizationmethod.

In operation 82, the syntax element encoder 12 classifies a secondcoordinate component of the last significant coefficient into a secondcoordinate prefix bit string and a second coordinate suffix bit stringbased on the critical value. As described above, the syntax elementencoder 12 binarizes the column location y according to a firstpredetermined binarization method and classifies a result of thebinarization as the y-axis prefix bit string when the column location yof the last significant coefficient is equal to or less than thecritical value th. Also, when the column location y of the lastsignificant coefficient exceeds the critical value th, the syntaxelement encoder 12 classifies the column location y into the y-axisprefix bit string that is obtained by performing binarization on thecritical value th according to the first binarization method and they-axis suffix bit string that is obtained by performing binarization ona value of (y-th) according to a second predetermined binarizationmethod. Here, when the width w and the height h of the block are equalto each other, the critical values that are criteria for classifying therow location x and the column location y into the prefixes and thesuffixes are equal to each other. If the block has a rectangular shape,the critical values for classifying the row location x and the columnlocation y into the prefixes and the suffixes may be determinedrespectively based on the width and the height of the block.

In operation 83, the syntax element encoder 12 sequentially performs thearithmetic encoding on the first coordinate prefix bit strings and thesecond coordinate prefix bit strings according to a first arithmeticencoding method. As described above, the syntax element encoder 12 maymake a group of the prefix bit strings to perform the arithmeticencoding by using the context model.

In operation 84, the syntax element encoder 12 performs the arithmeticencoding on the first coordinate suffix bit strings and the secondcoordinate suffix bit strings according to a second arithmetic encodingmethod. As described above, the syntax element encoder 12 may make agroup of the suffix bit strings that are arithmetically encoded in thebypass model.

FIG. 9A is a block diagram of a video decoding apparatus 90 according toan embodiment of the present invention.

Referring to FIG. 9A, the video decoding apparatus includes a syntaxelement decoder 91 and an image restoring unit 92.

The syntax element decoder 91 receives a bit stream including varioussyntax elements representing encoded data of the video, and parses thebit stream to acquire the syntax elements. The above operations may beperformed in a parsing unit of the syntax element decoder 91. Since thesyntax elements are binarized and arithmetically encoded by the abovedescribed video encoding apparatus 10, the syntax element decoder 91restores the syntax elements through arithmetically decoding andinverse-binarization processes.

In particular, the syntax element decoder 91 of the present embodimentacquires an x-axis component prefix syntax element and an x-axiscomponent suffix syntax element respectively corresponding to the x-axiscomponent prefix bit string and the x-axis component suffix bit stringthat are obtained by classifying the x-axis location of the lastsignificant coefficient of the current block based on the critical valuedetermined based on the size of the current block, and a y-axiscomponent prefix syntax element and a y-axis component suffix syntaxelement respectively corresponding to the y-axis component prefix bitstring and the y-axis component suffix bit string that are obtained byclassifying the y-axis location of the last significant coefficientbased on the critical value, from the received bit stream.

In addition, the syntax element decoder 91 performs arithmetic decodingoperations sequentially with respect to the x-axis component prefixsyntax element and the y-axis component prefix syntax element in a firstarithmetic decoding method, that is, the arithmetic decoding using thecontext model, in order to acquire the x-axis component prefix bitstring and the y-axis component prefix bit string.

Also, the syntax element decoder 91 performs arithmetic decodingoperations sequentially with respect to the x-axis component suffixsyntax element and the y-axis component suffix syntax element in asecond arithmetic decoding method, that is, the arithmetic decoding ofthe bypass mode, in order to acquire the x-axis component suffix bitstring and the y-axis component suffix bit string. As described above,since there is no suffix in a case where the value of the row location xand the column location y is less than the predetermined critical valueth, the arithmetic decoding operation of the suffix syntax element isomitted in this case.

When the x-axis component prefix bit string, the y-axis component prefixbit string, the x-axis component suffix bit string, and the y-axiscomponent suffix bit string are acquired, the syntax element decoderperforms inverse binarization on the x-axis component prefix bit stringand the x-axis component suffix bit string, and adds the inverselybinarized x-axis component prefix and the x-axis component suffix torecover the x-axis component. Also, the syntax element decoder 91performs inverse binarization on the y-axis component prefix bit stringand the y-axis component suffix bit string, and adds the inverselybinarized y-axis component prefix and the y-axis component suffix torecover the y-axis component.

The image restoring unit 92 performs inverse transformation andprediction on the current block by using various syntax elementsrestored by the syntax element decoder 91. The image restoring unit 92may restore the image blocks by performing operations such as inversequantization, inverse transformation, and intra prediction/motioncompensation by using the restored syntax elements in each of the imageblocks.

FIG. 9B is a block diagram of an arithmetic decoding apparatus includedin the syntax element decoder 91 of FIG. 9A. The arithmetic decodingapparatus 93 shown in FIG. 9B corresponds to the syntax element encodingapparatus 20 shown in FIG. 2. The arithmetic decoding apparatus 93performs processes that are the inverse of the arithmetic encodingprocesses performed in the syntax element encoding apparatus 20.

Referring to FIG. 9B, the arithmetic decoding apparatus 93 includes acontext modeller 94, a regular decoder 95, a bypass decoder 95, and aninverse binarization unit 97.

Symbols encoded in the bypass mode are output to the bypass decoder 96to be decoded, and symbols encoded in a regular coding method aredecoded by the regular decoder 95. The regular decoder 95 arithmeticallydecodes a binarization value of the currently encoded symbol based on acontext model provided from the context modeller 94. As described above,the regular decoder 95 performs the arithmetic decoding using thecontext model sequentially with respect to the x-axis component prefixsyntax element and the y-axis component prefix syntax element acquiredfrom the received bit stream, and thus, acquires the x-axis componentprefix bit string and the y-axis component prefix bit string.

The bypass decoder 96 performs the arithmetic decoding in the bypassmode with respect to the x-axis component suffix syntax element and they-axis component suffix syntax element acquired from the received bitstream, and thus, acquires the x-axis component suffix bit string andthe y-axis component suffix bit string.

The inverse binarization unit 97 performs inverse binarization on thex-axis component prefix bit string, the y-axis component prefix bitstring, the x-axis component suffix bit string, and the y-axis componentsuffix bit string recovered by the regular decoder 95 or the bypassdecoder 96 in order to restore the x-axis component prefix, the y-axiscomponent prefix, the x-axis component suffix, and the y-axis componentsuffix. The x-axis component prefix and the x-axis component suffix areadded to obtain the x-axis location of the last significant coefficientand the y-axis component prefix and the y-axis component suffix areadded to obtain the y-axis location of the last significant coefficient.

FIG. 10 is a flowchart illustrating an arithmetic decoding method fordecoding video, according to an embodiment of the present invention.

Referring to FIG. 10, in operation 101, the syntax element decoder 91acquires a first coordinate component prefix syntax element and a firstcoordinate component suffix syntax element respectively corresponding tothe first coordinate component prefix bit string and the firstcoordinate component suffix bit string that are obtained by classifyingthe first coordinate location of the last significant coefficient of thecurrent block based on the critical value determined based on the sizeof the current block, and a second coordinate component prefix syntaxelement and a second coordinate component suffix syntax elementrespectively corresponding to the second coordinate component prefix bitstring and the second coordinate component suffix bit string that areobtained by classifying the second coordinate location of the lastsignificant coefficient based on the critical value, from the receivedbit stream.

In operation 102, the syntax element decoder 91 performs arithmeticdecoding operations sequentially with respect to the first coordinatecomponent prefix syntax element and the second coordinate componentprefix syntax element in a first arithmetic decoding method, that is,the arithmetic decoding using the context model, in order to acquire thefirst coordinate component prefix bit string and the second coordinatecomponent prefix bit string.

In operation 103, the syntax element decoder 91 performs arithmeticdecoding operations sequentially with respect to the first coordinatecomponent suffix syntax element and the second coordinate componentsuffix syntax element in a second arithmetic decoding method, that is,the arithmetic decoding of the bypass mode, in order to acquire thefirst coordinate component suffix bit string and the second coordinatecomponent suffix bit string.

In operation 104, the syntax element decoder 91 performs inversebinarization on the first coordinate prefix bit string and the firstcoordinate suffix bit string to restore the first coordinate component,and performs inverse binarization on the second coordinate prefix bitstring and the second coordinate suffix bit string to restore the secondcoordinate component.

According to the above embodiment of the present invention, the locationof the last significant coefficient is classified into the prefix bitstrings and the suffix bit strings, and the bit strings to which thesame arithmetic encoding method is applied are configured as a group tobe processed sequentially. The prefix bit strings are arithmeticallyencoded and decoded by using the context modelling, and the suffix bitstrings are arithmetically encoded and decoded in the bypass modewithout using the context modelling. Thus, a calculation amount isreduced when compared with a case where the location of the lastsignificant coefficient is arithmetically encoded and decoded by usingthe context modelling only, and thus, processing speed of the arithmeticencoding and decoding operations may be improved.

As described above, in the video encoding apparatus 10 and the videodecoding apparatus 90 according to the present embodiment, the blocksobtained by splitting the video data are split into coding units havinga tree structure, and prediction units are used to predict the codingunits and transformation units are used to transform the coding units.Hereinafter, a video encoding method and apparatus and a video decodingmethod and apparatus based on the coding units, the prediction units,and the transformation units having tree structures will be describedwith reference to FIGS. 11 through 23 below.

FIG. 11 is a block diagram of a video encoding apparatus 100 accordingto an embodiment of the present invention.

The video encoding apparatus 100 includes a hierarchical encoder 110 andan entropy encoder 120.

The hierarchical encoder 110 splits a current picture that is encodedinto data units of predetermined sizes to perform an encoding operationin each of the data units. In particular, the hierarchical encoder 110may split a current picture based on a maximum coding unit that is acoding unit of a largest size. The maximum coding unit according to anembodiment of the present invention may be a data unit having a size of32×32, 64×64, 128×128, 256×256, etc., wherein a shape of the data unitis a square having a width and length in squares of 2.

A coding unit according to an embodiment of the present invention may becharacterized by a maximum size and a depth. The depth denotes a numberof times the coding unit is spatially split from the maximum codingunit, and as the depth deepens, deeper encoding units according todepths may be split from the maximum coding unit to a minimum codingunit. A depth of the maximum coding unit is an uppermost depth and adepth of the minimum coding unit is a lowermost depth. Since a size of acoding unit corresponding to each depth decreases as the depth of themaximum coding unit deepens, a coding unit corresponding to an upperdepth may include a plurality of coding units corresponding to lowerdepths.

As described above, the image data of the current picture is split intothe maximum coding units according to a maximum size of the coding unit,and each of the maximum coding units may include deeper coding unitsthat are split according to depths. Since the maximum coding unitaccording to an embodiment of the present invention is split accordingto depths, the image data of a spatial domain included in the maximumcoding unit may be hierarchically classified according to depths.

A maximum depth and a maximum size of a coding unit, which limit thetotal number of times a height and a width of the maximum coding unitare hierarchically split may be predetermined.

The hierarchical encoder 110 encodes at least one split region obtainedby splitting a region of the maximum coding unit according to depths,and determines a depth to output finally encoded image data according tothe at least one split region.

In other words, the hierarchical encoder 110 determines a coded depth byencoding the image data in the deeper coding units according to depths,according to the maximum coding unit of the current picture, andselecting a depth having the least encoding error. The determined codeddepth and the encoded image data according to the determined coded depthare output to the entropy encoder 120.

The image data in the maximum coding unit is encoded based on the deepercoding units corresponding to at least one depth equal to or smallerthan the maximum depth, and results of encoding the image data arecompared based on each of the deeper coding units. A depth having theleast encoding error may be selected after comparing encoding errors ofthe deeper coding units. At least one coded depth may be selected foreach maximum coding unit.

The size of the maximum coding unit is split as a coding unit ishierarchically split according to depths, and as the number of codingunits increases. Also, even if coding units correspond to a same depthin one maximum coding unit, it is determined whether to split each ofthe coding units corresponding to the same depth to a lower depth bymeasuring an encoding error of the image data of the each coding unit,separately. Accordingly, even when image data is included in one maximumcoding unit, the image data is split into regions according to thedepths and the encoding errors may differ according to regions in theone maximum coding unit, and thus the coded depths may differ accordingto regions in the image data. Thus, one or more coded depths may bedetermined in one maximum coding unit, and the image data of the maximumcoding unit may be divided according to coding units of at least onecoded depth.

Accordingly, the hierarchical encoder 110 may determine coding unitshaving a tree structure included in the maximum coding unit. The ‘codingunits having a tree structure’ according to an embodiment of the presentinvention include coding units corresponding to a depth determined to bethe coded depth, from among all deeper coding units included in themaximum coding unit. A coding unit of a coded depth may behierarchically determined according to depths in the same region of themaximum coding unit, and may be independently determined in differentregions. Similarly, a coded depth in a current region may beindependently determined from a coded depth in another region.

A maximum depth according to an embodiment of the present invention isan index related to the number of times splitting is performed from amaximum coding unit to a minimum coding unit. A first maximum depthaccording to an embodiment of the present invention may denote the totalnumber of times splitting is performed from the maximum coding unit tothe minimum coding unit. A second maximum depth according to anembodiment of the present invention may denote the total number of depthlevels from the maximum coding unit to the minimum coding unit. Forexample, when a depth of the maximum coding unit is 0, a depth of acoding unit, in which the maximum coding unit is split once, may be setto 1, and a depth of a coding unit, in which the maximum coding unit issplit twice, may be set to 2. Here, if the minimum coding unit is acoding unit in which the maximum coding unit is split four times, 5depth levels of depths 0, 1, 2, 3 and 4 exist, and thus the firstmaximum depth may be set to 4, and the second maximum depth may be setto 5.

Prediction encoding and transformation may be performed according to themaximum coding unit. The prediction encoding and the transformation arealso performed based on the deeper coding units according to a depthequal to or depths less than the maximum depth, according to the maximumcoding unit.

Since the number of deeper coding units increases whenever the maximumcoding unit is split according to depths, encoding including theprediction encoding and the transformation is performed on all of thedeeper coding units generated as the depth deepens. For convenience ofdescription, the prediction encoding and the transformation will now bedescribed based on a coding unit of a current depth, in a maximum codingunit.

The video encoding apparatus 100 may variously select a size or shape ofa data unit for encoding the image data. In order to encode the imagedata, operations, such as prediction encoding, transformation, andentropy encoding, are performed, and at this time, the same data unitmay be used for all operations or different data units may be used foreach operation.

For example, the video encoding apparatus 100 may select not only acoding unit for encoding the image data, but also a data unit differentfrom the coding unit so as to perform the prediction encoding on theimage data in the coding unit.

In order to perform prediction encoding in the maximum coding unit, theprediction encoding may be performed based on a coding unitcorresponding to a coded depth, i.e., based on a coding unit that is nolonger split into coding units corresponding to a lower depth.Hereinafter, the coding unit that is no longer split and becomes a basisunit for prediction encoding will now be referred to as a ‘predictionunit’. A partition obtained by splitting the prediction unit may includea prediction unit or a data unit obtained by splitting at least one of aheight and a width of the prediction unit.

For example, when a coding unit of 2N×2N (where N is a positive integer)is no longer split and becomes a prediction unit of 2N×2N, and a size ofa partition may be 2N×2N, 2N×N, N×2N, or N×N. Examples of a partitiontype include symmetrical partitions that are obtained by symmetricallysplitting a height or width of the prediction unit, partitions obtainedby asymmetrically splitting the height or width of the prediction unit,such as 1:n or n:1, partitions that are obtained by geometricallysplitting the prediction unit, and partitions having arbitrary shapes.

A prediction mode of the prediction unit may be at least one of an intramode, a inter mode, and a skip mode. For example, the intra mode or theinter mode may be performed on the partition of 2N×2N, 2N×N, N×2N, orN×N. Also, the skip mode may be performed only on the partition of2N×2N. The encoding is independently performed on one prediction unit ina coding unit, thereby selecting a prediction mode having a leastencoding error.

The video encoding apparatus 100 may also perform the transformation onthe image data in a coding unit based not only on the coding unit forencoding the image data, but also based on a data unit that is differentfrom the coding unit.

In order to perform the transformation in the coding unit, thetransformation may be performed based on a data unit having a sizesmaller than or equal to the coding unit. For example, the data unit forthe transformation may include a data unit for an intra mode and a dataunit for an inter mode.

A data unit used as a base of the transformation will now be referred toas a ‘transformation unit’. Similarly to the coding unit, thetransformation unit in the coding unit may be recursively split intosmaller sized regions, so that the transformation unit may be determinedindependently in units of regions. Thus, residual data in the codingunit may be divided according to the transformation unit having the treestructure according to transformation depths.

A transformation depth indicating the number of times splitting isperformed to reach the transformation unit by splitting the height andwidth of the coding unit may also be set in the transformation unit. Forexample, in a current coding unit of 2N×2N, a transformation depth maybe 0 when the size of a transformation unit is 2N×2N, may be 11 when thesize of a transformation unit is N×N, and may be 12 when the size of atransformation unit is N/2×N/2. That is, the transformation unit havingthe tree structure may also be set according to transformation depths.

Encoding information according to coding units corresponding to a codeddepth requires not only information about the coded depth, but alsoabout information related to prediction encoding and transformation.Accordingly, the hierarchical encoder 110 not only determines a codeddepth having a least encoding error, but also determines a partitiontype in a prediction unit, a prediction mode according to predictionunits, and a size of a transformation unit for transformation.

Coding units according to a tree structure in a maximum coding unit anda method of determining a partition, according to embodiments of thepresent invention, will be described in detail later.

The hierarchical encoder 110 may measure an encoding error of deepercoding units according to depths by using Rate-Distortion Optimizationbased on Lagrangian multipliers.

The entropy encoder 120 outputs the image data of the maximum codingunit, which is encoded based on the at least one coded depth determinedby the hierarchical encoder 110, and information about the encoding modeaccording to the coded depth, in bitstreams. The encoded image data maybe obtained by encoding residual data of an image. The information aboutthe encoding mode according to coded depth may include information aboutthe coded depth, about the partition type in the prediction unit, theprediction mode, and the size of the transformation unit.

The information about the coded depth may be defined by using splitinformation according to depths, which indicates whether encoding isperformed on coding units of a lower depth instead of a current depth.If the current depth of the current coding unit is the coded depth,image data in the current coding unit is encoded and output, and thusthe split information may be defined not to split the current codingunit to a lower depth. Alternatively, if the current depth of thecurrent coding unit is not the coded depth, the encoding is performed onthe coding unit of the lower depth, and thus the split information maybe defined to split the current coding unit to obtain the coding unitsof the lower depth.

If the current depth is not the coded depth, encoding is performed onthe coding unit that is split into the coding unit of the lower depth.Since at least one coding unit of the lower depth exists in one codingunit of the current depth, the encoding is repeatedly performed on eachcoding unit of the lower depth, and thus the encoding may be recursivelyperformed for the coding units having the same depth.

Since the coding units having a tree structure are determined for onemaximum coding unit, and information about at least one encoding mode isdetermined for a coding unit of a coded depth, information about atleast one encoding mode may be determined for one maximum coding unit.Also, a coded depth of the image data of the maximum coding unit may bedifferent according to locations since the image data is hierarchicallysplit according to depths, and thus information about the coded depthand the encoding mode may be set for the image data.

Accordingly, the entropy encoder 120 may assign encoding informationabout a corresponding coded depth and an encoding mode to at least oneof the coding unit, the prediction unit, and a minimum unit included inthe maximum coding unit.

The minimum unit according to an embodiment of the present invention isa square data unit obtained by splitting the minimum coding unitconstituting the lowermost depth by 4. Alternatively, the minimum unitmay be a maximum square data unit that may be included in all of thecoding units, prediction units, partition units, and transformationunits included in the maximum coding unit.

For example, the encoding information output through the entropy encoder120 may be classified into encoding information according to codingunits, and encoding information according to prediction units. Theencoding information according to the coding units may include theinformation about the prediction mode and about the size of thepartitions. The encoding information according to the prediction unitsmay include information about an estimated direction of an inter mode,about a reference image index of the inter mode, about a motion vector,about a chroma component of an intra mode, and about an interpolationmethod of the intra mode. Also, information about a maximum size of thecoding unit defined according to pictures, slices, or GOPs, andinformation about a maximum depth may be inserted into a header of abitstream.

In the video encoding apparatus 100, the deeper coding unit may be acoding unit obtained by dividing a height or width of a coding unit ofan upper depth, which is one layer above, by two. In other words, whenthe size of the coding unit of the current depth is 2N×2N, the size ofthe coding unit of the lower depth is N×N. Also, the coding unit of thecurrent depth having the size of 2N×2N may include a maximum number of 4coding units of the lower depth having the size of N×N.

Accordingly, the video encoding apparatus 100 may form the coding unitshaving the tree structure by determining coding units having an optimumshape and an optimum size for each maximum coding unit, based on thesize of the maximum coding unit and the maximum depth determinedconsidering characteristics of the current picture. Also, since encodingmay be performed on each maximum coding unit by using any one of variousprediction modes and transformations, an optimum encoding mode may bedetermined considering characteristics of the coding unit of variousimage sizes.

Thus, if an image having high resolution or a large data amount isencoded in a conventional macroblock, a number of macroblocks perpicture excessively increases. Accordingly, a number of pieces ofcompressed information generated for each macroblock increases, and thusit is difficult to transmit the compressed information and datacompression efficiency decreases. However, by using the video encodingapparatus 100, image compression efficiency may be increased since acoding unit is adjusted while considering characteristics of an imagewhile increasing a maximum size of a coding unit while considering asize of the image.

FIG. 12 is a block diagram of a video decoding apparatus 200, accordingto an embodiment of the present invention.

The video decoding apparatus 200 includes a syntax element extractor210, an entropy decoder 220, and a hierarchical decoder 230. Definitionsof various terms, such as a coding unit, a depth, a prediction unit, atransformation unit, and information about various encoding modes, forvarious operations of the video decoding apparatus 200 are identical tothose described with reference to FIG. 11 and the video encodingapparatus 100.

The syntax element extractor 210 receives and parses a bitstream of anencoded video. The entropy decoder 220 extracts encoded image data foreach coding unit from the parsed bitstream, wherein the coding unitshave a tree structure according to each maximum coding unit, and outputsthe extracted image data to the hierarchical decoder 230.

Also, the entropy decoder 220 extracts additional information about acoded depth, an encoding mode, color component information, andprediction mode information for the coding units having a tree structureaccording to each maximum coding unit, from the parsed bitstream. Theextracted information about the coded depth and the encoding mode isoutput to the hierarchical decoder 230. In other words, the image datain a bit stream is split into the maximum coding unit so that thehierarchical decoder 230 decodes the image data for each maximum codingunit.

The information about the coded depth and the encoding mode according tothe maximum coding unit extracted by the entropy decoder 220 may be setfor information about the coded depth and the encoding mode that aredetermined to generate minimum encoding error by performing the encodingrepeatedly in each of coding units according to the maximum coding unitand the coded depth at an encoding terminal, like the video encodingapparatus 100 of the embodiment. Therefore, the video decoding apparatus200 may restore the image by decoding the data according to the encodingmethod generating the minimum encoding error.

Since encoding information about the coded depth and the encoding modemay be assigned to a predetermined data unit from among a correspondingcoding unit, a prediction unit, and a minimum unit, the entropy decoder220 may extract the information about the coded depth and the encodingmode according to the predetermined data units. If the information aboutthe coded depth and the encoding mode of the corresponding maximumcoding unit is recorded in each of predetermined data units, thepredetermined data units to which the same information about the codeddepth and the encoding mode is assigned may be inferred to be the dataunits included in the same maximum coding unit.

The hierarchical decoder 230 restores the current picture by decodingthe image data in each maximum coding unit based on the informationabout the coded depth and the encoding mode according to the maximumcoding units. In other words, the hierarchical decoder 230 may decodethe encoded image data based on the extracted information about thepartition type, the prediction mode, and the transformation unit foreach coding unit from among the coding units having the tree structureincluded in each maximum coding unit. A decoding process may includeprediction including intra prediction and motion compensation, andinverse transformation. Inverse transformation may be performedaccording to a method of inverse orthogonal transformation or inverseinteger transformation.

The hierarchical decoder 230 may perform intra prediction or motioncompensation according to a partition and a prediction mode of eachcoding unit, based on the information about the partition type and theprediction mode of the prediction unit of the coding unit according tocoded depths.

Also, the hierarchical decoder 230 may perform inverse transformationaccording to each transformation unit in the coding unit, based on theinformation about the size of the transformation unit of the coding unitaccording to coded depths, so as to perform the inverse transformationaccording to maximum coding units.

The hierarchical decoder 230 may determine at least one coded depth of acurrent maximum coding unit by using split information according todepths. If the split information indicates that image data is no longersplit in the current depth, the current depth is a coded depth.Accordingly, the hierarchical decoder 230 may decode encoded data of atleast one coding unit corresponding to the each coded depth in thecurrent maximum coding unit by using the information about the partitiontype of the prediction unit, the prediction mode, and the size of thetransformation unit for each coding unit corresponding to the codeddepth, and output the image data of the current maximum coding unit.

In other words, data units containing the encoding information includingthe same split information may be gathered by observing the encodinginformation set assigned for the predetermined data unit from among thecoding unit, the prediction unit, and the minimum unit, and the gathereddata units may be considered to be one data unit to be decoded by thehierarchical decoder 230 in the same encoding mode.

The video decoding apparatus 200 may obtain information about at leastone coding unit that generates the minimum encoding error when encodingis recursively performed for each maximum coding unit, and may use theinformation to decode the current picture. In other words, the codingunits having the tree structure determined to be the optimum codingunits in each maximum coding unit may be decoded.

Accordingly, even if image data has high resolution and a large amountof data, the image data may be efficiently decoded and restored by usinga size of a coding unit and an encoding mode, which are adaptivelydetermined according to characteristics of the image data, by usinginformation about an optimum encoding mode received from an encoder.

A method of determining coding units having a tree structure, aprediction unit, and a transformation unit, according to an embodimentof the present invention, will now be described with reference to FIGS.13 through 23.

FIG. 3 is a diagram for describing a concept of coding units accordingto an embodiment of the present invention.

A size of a coding unit may be expressed in width x height, and may be64×64, 32×32, 16×16, and 8×8. A coding unit of 64×64 may be split intopartitions of 64×64, 64×32, 32×64, or 32×32, and a coding unit of 32×32may be split into partitions of 32×32, 32×16, 16×32, or 16×16, a codingunit of 16×16 may be split into partitions of 16×16, 16×8, 8×16, or 8×8,and a coding unit of 8×8 may be split into partitions of 8×8, 8×4, 4×8,or 4×4.

In video data 310, a resolution is 1920×1080, a maximum size of a codingunit is 64, and a maximum depth is 2. In video data 320, a resolution is1920×1080, a maximum size of a coding unit is 64, and a maximum depth is3. In video data 330, a resolution is 352×288, a maximum size of acoding unit is 16, and a maximum depth is 1. The maximum depth shown inFIG. 13 denotes a total number of splits from a maximum coding unit to aminimum decoding unit.

If a resolution is high or a data amount is large, a maximum size of acoding unit may be large so as to not only increase encoding efficiencybut also to accurately reflect characteristics of an image. Accordingly,the maximum size of the coding unit of the video data 310 and 320 havingthe higher resolution than the video data 330 may be 64.

Since the maximum depth of the video data 310 is 2, coding units 315 ofthe vide data 310 may include a maximum coding unit having a long axissize of 64, and coding units having long axis sizes of 32 and 16 sincedepths are deepened to two layers by splitting the maximum coding unittwice. Meanwhile, since the maximum depth of the video data 330 is 1,coding units 335 of the video data 330 may include a maximum coding unithaving a long axis size of 16, and coding units having a long axis sizeof 8 since depths are deepened to one layer by splitting the maximumcoding unit once.

Since the maximum depth of the video data 320 is 3, coding units 325 ofthe video data 320 may include a maximum coding unit having a long axissize of 64, and coding units having long axis sizes of 32, 16, and 8since the depths are deepened to 3 layers by splitting the maximumcoding unit three times. As a depth deepens, detailed information may beprecisely expressed.

FIG. 14 is a block diagram of an image encoding apparatus based oncoding units of a hierarchical structure, according to an embodiment ofthe present invention.

An intra predictor 410 performs intra prediction on coding units in anintra mode, from among a current frame 405, and a motion estimator 420and a motion compensator 425 performs inter estimation and motioncompensation on coding units in an inter mode from among the currentframe 405 by using the current frame 405, and a reference frame 495.

Data output from the intra predictor 410, the motion estimator 420, andthe motion compensator 425 is output as a quantized transformationcoefficient through a transformer 430 and a quantizer 440. The quantizedtransformation coefficient is restored as data in a spatial domainthrough an inverse quantizer 460 and an inverse transformer 470, and therestored data in the spatial domain is output as the reference frame 495after being post-processed through a deblocking unit 480 and a loopfiltering unit 490. The quantized transformation coefficient may beoutput as a bitstream 455 through an entropy encoder 450.

In order for the image encoder 400 to be applied in the video encodingapparatus 100, all elements of the image encoder 400, i.e., the intrapredictor 410, the motion estimator 420, the motion compensator 425, thetransformer 430, the quantizer 440, the entropy encoder 450, an inversequantizer 460, an inverse transformer 470, a deblocking unit 480, and aloop filtering unit 490 perform operations based on each coding unitfrom among coding units having a tree structure while considering themaximum depth of each maximum coding unit.

Specifically, the intra predictor 410, the motion estimator 420, and themotion compensator 425 determines partitions and a prediction mode ofeach coding unit from among the coding units having a tree structurewhile considering the maximum size and the maximum depth of a currentmaximum coding unit, and the transformer 430 determines the size of thetransformation unit in each coding unit from among the coding unitshaving a tree structure.

FIG. 15 is a block diagram of an image decoding apparatus based oncoding units of a hierarchical structure, according to an embodiment ofthe present invention.

A parser 510 parses encoded image data to be decoded and informationabout encoding required for decoding from a bitstream 505. The encodedimage data is output as inverse quantized data through an entropydecoder 520 and an inverse quantizer 530, and the inverse quantized datais restored to image data in a spatial domain through an inversetransformer 540.

An intra predictor 550 performs intra prediction on coding units in anintra mode with respect to the image data in the spatial domain, and amotion compensator 560 performs motion compensation on coding units inan inter mode by using a reference frame 585.

The image data in the spatial domain, which passed through the intrapredictor 550 and the motion compensator 560, may be output as arestored frame 595 after being post-processed through a deblocking unit570 and a loop filtering unit 580. Also, the image data, which ispost-processed through the deblocking unit 570 and the loop filteringunit 580, may be output as the reference frame 585.

In order for the image decoder 500 to be applied in the video decodingapparatus 200, all elements of the image decoder 500, i.e., the parser510, the entropy decoder 520, the inverse quantizer 530, the inversetransformer 540, the intra predictor 550, the motion compensator 560,the deblocking unit 570, and the loop filtering unit 580 performoperations based on coding units having a tree structure for eachmaximum coding unit.

Specifically, the intra prediction 550 and the motion compensator 560perform operations based on partitions and a prediction mode for each ofthe coding units having a tree structure, and the inverse transformer540 perform operations based on a size of a transformation unit for eachcoding unit.

FIG. 16 is a diagram illustrating coding units according to depths, andpartitions, according to an embodiment of the present invention.

The video encoding apparatus 100 and the video decoding apparatus 200use hierarchical coding units so as to consider characteristics of animage. A maximum height, a maximum width, and a maximum depth of codingunits may be adaptively determined according to the characteristics ofthe image, or may be differently set by a user. Sizes of deeper codingunits according to depths may be determined according to thepredetermined maximum size of the coding unit.

In a hierarchical structure 600 of coding units, according to anembodiment of the present invention, the maximum height and the maximumwidth of the coding units are each 64, and the maximum depth is 4. Sincea depth deepens along a vertical axis of the hierarchical structure 600,a height and a width of the deeper coding unit are each split. Also, aprediction unit and partitions, which are bases for prediction encodingof each deeper coding unit, are shown along a horizontal axis of thehierarchical structure 600.

In other words, a coding unit 610 is a maximum coding unit in thehierarchical structure 600, wherein a depth is 0 and a size, i.e., aheight by width, is 64×64. The depth deepens along the vertical axis,and a coding unit 620 having a size of 32×32 and a depth of 1, a codingunit 630 having a size of 16×16 and a depth of 2, a coding unit 640having a size of 8×8 and a depth of 3, and a coding unit 650 having asize of 4×4 and a depth of 4 exist. The coding unit 650 having the sizeof 4×4 and the depth of 4 is a minimum coding unit.

The prediction unit and the partitions of a coding unit are arrangedalong the horizontal axis according to each depth. In other words, ifthe coding unit 610 having the size of 64×64 and the depth of 0 is aprediction unit, the prediction unit may be split into partitionsinclude in the encoding unit 610, i.e. a partition 610 having a size of64×64, partitions 612 having the size of 64×32, partitions 614 havingthe size of 32×64, or partitions 616 having the size of 32×32.

Similarly, a prediction unit of the coding unit 620 having the size of32×32 and the depth of 11 may be split into partitions included in thecoding unit 620, i.e. a partition 620 having a size of 32×32, partitions622 having a size of 32×16, partitions 624 having a size of 16×32, andpartitions 626 having a size of 16×16.

Similarly, a prediction unit of the coding unit 630 having the size of16×16 and the depth of 12 may be split into partitions included in thecoding unit 630, i.e. a partition having a size of 16×16 included in thecoding unit 630, partitions 632 having a size of 16×8, partitions 634having a size of 8×16, and partitions 636 having a size of 8×8.

Similarly, a prediction unit of the coding unit 640 having the size of8×8 and the depth of 23 may be split into partitions included in thecoding unit 640, i.e. a partition having a size of 8×8 included in thecoding unit 640, partitions 642 having a size of 8×4, partitions 644having a size of 4×8, and partitions 646 having a size of 4×4.

The coding unit 650 having the size of 4×4 and the depth of 14 is theminimum coding unit and a coding unit of the lowermost depth. Aprediction unit of the coding unit 650 is only assigned to a partitionhaving a size of 4×4.

In order to determine the at least one coded depth of the coding unitsconstituting the maximum coding unit 610, the coding unit determiner 120of the video encoding apparatus 100 performs encoding for coding unitscorresponding to each depth included in the maximum coding unit 610.

A number of deeper coding units according to depths including data inthe same range and the same size increases as the depth deepens. Forexample, four coding units corresponding to a depth of 12 are requiredto cover data that is included in one coding unit corresponding to adepth of 11. Accordingly, in order to compare encoding results of thesame data according to depths, the coding unit corresponding to thedepth of 11 and four coding units corresponding to the depth of 12 areeach encoded.

In order to perform encoding for a current depth from among the depths,a least encoding error may be selected for the current depth byperforming encoding for each prediction unit in the coding unitscorresponding to the current depth, along the horizontal axis of thehierarchical structure 600. Alternatively, the minimum encoding errormay be searched for by comparing the least encoding errors according todepths and performing encoding for each depth as the depth deepens alongthe vertical axis of the hierarchical structure 600. A depth and apartition having the minimum encoding error in the coding unit 610 maybe selected as the coded depth and a partition type of the coding unit610.

FIG. 17 is a diagram for describing a relationship between a coding unit710 and transformation units 720, according to an embodiment of thepresent invention.

The video encoding apparatus 100 or 200 encodes or decodes an imageaccording to coding units having sizes smaller than or equal to amaximum coding unit for each maximum coding unit. Sizes oftransformation units for transformation during encoding may be selectedbased on data units that are not larger than a corresponding codingunit.

For example, in the video encoding apparatus 100 or 200, if a size ofthe coding unit 710 is 64×64, transformation may be performed by usingthe transformation units 720 having a size of 32×32.

Also, data of the coding unit 710 having the size of 64×64 may beencoded by performing the transformation on each of the transformationunits having the size of 32×32, 16×16, 8×8, and 4×4, which are smallerthan 64×64, and then a transformation unit having the least coding errormay be selected.

FIG. 18 is a diagram for describing encoding information of coding unitscorresponding to a coded depth, according to an embodiment of thepresent invention.

The output unit 130 of the video encoding apparatus 100 may encode andtransmit information 800 about a partition type, information 810 about aprediction mode, and information 820 about a size of a transformationunit for each coding unit corresponding to a coded depth, as informationabout an encoding mode.

The information 800 indicates information about a shape of a partitionobtained by splitting a prediction unit of a current coding unit,wherein the partition is a data unit for prediction encoding the currentcoding unit. For example, a current coding unit CU_0 having a size of2N×2N may be split into any one of a partition 802 having a size of2N×2N, a partition 804 having a size of 2N×N, a partition 806 having asize of N×2N, and a partition 808 having a size of N×N. Here, theinformation 800 about a partition type is set to indicate one of thepartition 804 having a size of 2N×N, the partition 806 having a size ofN×2N, and the partition 808 having a size of N×N.

The information 810 indicates a prediction mode of each partition. Forexample, the information 810 may indicate a mode of prediction encodingperformed on a partition indicated by the information 800, i.e., anintra mode 812, an inter mode 814, or a skip mode 816.

The information 820 indicates a transformation unit to be based on whentransformation is performed on a current coding unit. For example, thetransformation unit may be a first intra transformation unit 822, asecond intra transformation unit 824, a first inter transformation unit826, or a second intra transformation unit 828.

The image data and encoding information extractor 220 of the videodecoding apparatus 200 may extract and use the information 800, 810, and820 for decoding, according to each deeper coding unit.

FIG. 19 is a diagram of deeper coding units according to depths,according to an embodiment of the present invention.

Split information may be used to indicate a change of a depth. The spiltinformation indicates whether a coding unit of a current depth is splitinto coding units of a lower depth.

A prediction unit 910 for prediction encoding a coding unit 900 having adepth of 0 and a size of 2N_0×2N_0 may include partitions of a partitiontype 912 having a size of 2N_0×2N_0, a partition type 914 having a sizeof 2N_0×N_0, a partition type 916 having a size of N_0×2N_0, and apartition type 918 having a size of N_0×N_0. FIG. 19 only illustratesthe partition types 912 through 918 which are obtained by symmetricallysplitting the prediction unit 910, but a partition type is not limitedthereto, and the partitions of the prediction unit 910 may includeasymmetrical partitions, partitions having a predetermined shape, andpartitions having a geometrical shape.

Prediction encoding is repeatedly performed on one partition having asize of 2N_0×2N_0, two partitions having a size of 2N_0×N_0, twopartitions having a size of N_0×2N_0, and four partitions having a sizeof N_0×N_0, according to each partition type. The prediction encoding inan intra mode and an inter mode may be performed on the partitionshaving the sizes of 2N_0×2N_0, N_0×2N_0, 2N_0×N_0, and N_0×N_0. Theprediction encoding in a skip mode is performed only on the partitionhaving the size of 2N_0×2N_0.

If an encoding error is smallest in one of the partition types 912through 916 having the sizes of 2N_0×2N_0, 2N_0×N_0, and N_0×2N_0, theprediction unit 910 may not be split into a lower depth.

If the encoding error is the smallest in the partition type 918 havingthe size of N_0×N_0, a depth is changed from 0 to 1 to split thepartition type 918 in operation 920, and encoding is repeatedlyperformed on coding units 930 having a depth of 12 and a size of N_0×N_0to search for a minimum encoding error.

A prediction unit 940 for prediction encoding the coding unit 930 havinga depth of 11 and a size of 2N_1×2N_1 (=N_0×N_0) may include partitionsof a partition type 942 having a size of 2N_1×2N_1, a partition type 944having a size of 2N_1×N_1, a partition type 946 having a size ofN_1×2N_1, and a partition type 948 having a size of N_1×N_1.

If an encoding error is the smallest in the partition type 948 havingthe size of N_1×N_1, a depth is changed from 11 to 12 to split thepartition type 948 in operation 950, and encoding is repeatedlyperformed on coding units 960, which have a depth of 2 and a size ofN_2×N_2 to search for a minimum encoding error.

When a maximum depth is d, split operation according to each depth maybe performed up to when a depth becomes d−1, and split information maybe encoded as up to when a depth is one of 0 to d−2. In other words,when encoding is performed up to when the depth is d−1 after a codingunit corresponding to a depth of d−2 is split in operation 970, aprediction unit 990 for prediction encoding a coding unit 980 having adepth of d−1 and a size of 2N_(d−1)×2N_(d−1) may include partitions of apartition type 992 having a size of 2N_(d−1)×2N_(d−1), a partition type994 having a size of 2N_(d−1)×N_(d−1), a partition type 996 having asize of N_(d−1)×2N_(d−1), and a partition type 998 having a size ofN_(d−1)×N_(d−1).

Prediction encoding may be repeatedly performed on one partition havinga size of 2N_(d−1)×2N_(d−1), two partitions having a size of2N_(d−1)×N_(d−1), two partitions having a size of N_(d−1)×2N_(d−1), fourpartitions having a size of N_(d−1)×N_(d−1) from among the partitiontypes 992 through 998 to search for a partition type having a minimumencoding error.

Even when the partition type 998 having the size of N_(d−1)×N_(d−1) hasthe minimum encoding error, since a maximum depth is d, a coding unitCU_(d−1) having a depth of d−1 is no longer split to a lower depth, anda coded depth for the coding units constituting a current maximum codingunit 900 is determined to be d−1 and a partition type of the currentmaximum coding unit 900 may be determined to be N_(d−1)×N_(d−1). Also,since the maximum depth is d and a minimum coding unit 980 having alowermost depth of d−1 is no longer split to a lower depth, splitinformation for the minimum coding unit 980 is not set.

A data unit 999 may be a ‘minimum unit’ for the current maximum codingunit. A minimum unit according to an embodiment of the present inventionmay be a rectangular data unit obtained by splitting a minimum codingunit 980 by 4. By performing the encoding repeatedly, the video encodingapparatus 100 may select a depth having the least encoding error bycomparing encoding errors according to depths of the coding unit 900 todetermine a coded depth, and set a corresponding partition type and aprediction mode as an encoding mode of the coded depth.

As such, the minimum encoding errors according to depths are compared inall of the depths of 1 through d, and a depth having the least encodingerror may be determined as a coded depth. The coded depth, the partitiontype of the prediction unit, and the prediction mode may be encoded andtransmitted as information about an encoding mode. Also, since a codingunit is split from a depth of 0 to a coded depth, only split informationof the coded depth is set to 0, and split information of depthsexcluding the coded depth is set to 1.

The image data and encoding information extractor 220 of the videodecoding apparatus 200 may extract and use the information about thecoded depth and the prediction unit of the coding unit 900 to decode thepartition 912. The video decoding apparatus 200 may determine a depth,in which split information is 0, as a coded depth by using splitinformation according to depths, and use information about an encodingmode of the corresponding depth for decoding.

FIGS. 20 through 22 are diagrams for describing a relationship betweencoding units 1010, prediction units 1060, and transformation units 1070,according to an embodiment of the present invention.

The coding units 1010 are coding units having a tree structure,corresponding to coded depths determined by the video encoding apparatus100, in a maximum coding unit. The prediction units 1060 are partitionsof prediction units of each of the coding units 1010, and thetransformation units 1070 are transformation units of each of the codingunits 1010.

When a depth of a maximum coding unit is 0 in the coding units 1010,depths of coding units 1012 and 1054 are 1, depths of coding units 1014,1016, 1018, 1028, 1050, and 1052 are 2, depths of coding units 1020,1022, 1024, 1026, 1030, 1032, and 1048 are 3, and depths of coding units1040, 1042, 1044, and 1046 are 4.

In the prediction units 1060, some encoding units 1014, 1016, 1022,1032, 1048, 1050, 1052, and 1054 are obtained by splitting the codingunits. In other words, partition types in the coding units 1014, 1022,1050, and 1054 have a size of 2N×N, partition types in the coding units1016, 1048, and 1052 have a size of N×2N, and a partition type of thecoding unit 1032 has a size of N×N. Prediction units and partitions ofthe coding units 1010 are smaller than or equal to each coding unit.

Transformation or inverse transformation is performed on image data ofthe coding unit 1052 in the transformation units 1070 in a data unitthat is smaller than the coding unit 1052. Also, the coding units 1014,1016, 1022, 1032, 1048, 1050, and 1052 in the transformation units 1070are different from those in the prediction units 1060 in terms of sizesand shapes. In other words, the video encoding and decoding apparatuses100 and 200 may perform intra prediction, motion estimation, motioncompensation, transformation, and inverse transformation individually ona data unit in the same coding unit.

Accordingly, encoding is recursively performed on each of coding unitshaving a hierarchical structure in each region of a maximum coding unitto determine an optimum coding unit, and thus coding units having arecursive tree structure may be obtained. Encoding information mayinclude split information about a coding unit, information about apartition type, information about a prediction mode, and informationabout a size of a transformation unit. Table 1 shows the encodinginformation that may be set by the video encoding and decodingapparatuses 100 and 200.

TABLE 1 Split Information 0 (Encoding on Coding Unit having Size of 2N ×2N and Current Depth of d) Size of Transformation Unit Split SplitPartition Type Information 0 Information 1 Symmetrical Asymmetrical ofof Prediction Partition Partition Transformation Transformation SplitMode Type Type Unit Unit Information 1 Intra 2N × 2N 2N × nU 2N × 2N N ×N Repeatedly Inter 2N × N  2N × nD (Symmetrical Encode Skip  N × 2N nL ×2N Type) Coding Units (Only N × N nR × 2N N/2 × N/2 having Lower 2N ×2N) (Asymmetrical Depth of d + 1 Type)

The entropy encoder 120 of the video encoding apparatus 100 may outputthe encoding information about the coding units having a tree structure,and the entropy decoder 220 of the video decoding apparatus 200 mayextract the encoding information about the coding units having a treestructure from a received bitstream.

Split information indicates whether a current coding unit is split intocoding units of a lower depth. If split information of a current depth dis 0, a depth, in which a current coding unit is no longer split into alower depth, is a coded depth, and thus information about a partitiontype, prediction mode, and a size of a transformation unit may bedefined for the coded depth. If the current coding unit is further splitaccording to the split information, encoding is independently performedon four split coding units of a lower depth.

A prediction mode may be one of an intra mode, an inter mode, and a skipmode. The intra mode and the inter mode may be defined in all partitiontypes, and the skip mode is defined only in a partition type having asize of 2N×2N.

The information about the partition type may indicate symmetricalpartition types having sizes of 2N×2N, 2N×N, N×2N, and N×N, which areobtained by symmetrically splitting a height or a width of a predictionunit, and asymmetrical partition types having sizes of 2N×nU, 2N×nD,nL×2N, and nR×2N, which are obtained by asymmetrically splitting theheight or width of the prediction unit. The asymmetrical partition typeshaving the sizes of 2N×nU and 2N×nD may be respectively obtained bysplitting the height of the prediction unit in 1:n (where n is aninteger greater than 1) and n:1, and the asymmetrical partition typeshaving the sizes of nL×2N and nR×2N may be respectively obtained bysplitting the width of the prediction unit in 1:n and n:1.

The size of the transformation unit may be set to be two types in theintra mode and two types in the inter mode. In other words, if splitinformation of the transformation unit is 0, the size of thetransformation unit may be 2N×2N, which is the size of the currentcoding unit. If split information of the transformation unit is 1, thetransformation units may be obtained by splitting the current codingunit. Also, if a partition type of the current coding unit having thesize of 2N×2N is a symmetrical partition type, a size of atransformation unit may be N×N, and if the partition type of the currentcoding unit is an asymmetrical partition type, the size of thetransformation unit may be N/2×N/2.

The encoding information about coding units having a tree structure mayinclude at least one of a coding unit corresponding to a coded depth, aprediction unit, and a minimum unit. The coding unit corresponding tothe coded depth may include at least one of a prediction unit and aminimum unit containing the same encoding information.

Accordingly, it is determined whether adjacent data units are includedin the same coding unit corresponding to the coded depth by comparingencoding information of the adjacent data units. Also, a correspondingcoding unit corresponding to a coded depth is determined by usingencoding information of a data unit, and thus a distribution of codeddepths in a maximum coding unit may be determined.

Accordingly, if a current coding unit is predicted based on encodinginformation of adjacent data units, encoding information of data unitsin deeper coding units adjacent to the current coding unit may bedirectly referred to and used.

Alternatively, if a current coding unit is predicted based on encodinginformation of adjacent data units, data units adjacent to the currentcoding unit are searched using encoded information of the data units,and the searched adjacent coding units may be referred to for predictingthe current coding unit.

FIG. 23 is a diagram for describing a relationship between a codingunit, a prediction unit, and a transformation unit, according to theencoding mode information of Table 1.

A maximum coding unit 1300 includes coding units 1302, 1304, 1306, 1312,1314, 1316, and 1318 of coded depths. Here, since the coding unit 1318is a coding unit of a coded depth, split information may be set to 0.Information about a partition type of the coding unit 1318 having a sizeof 2N×2N may be set to be one of a partition type 1322 having a size of2N×2N, a partition type 1324 having a size of 2N×N, a partition type1326 having a size of N×2N, a partition type 1328 having a size of N×N,a partition type 1332 having a size of 2N×nU, a partition type 1334having a size of 2N×nD, a partition type 1336 having a size of nL×2N,and a partition type 1338 having a size of nR×2N.

When the partition type is set to be symmetrical, i.e. the partitiontype 1322, 1324, 1326, or 1328, a transformation unit 1342 having a sizeof 2N×2N is set if split information (TU size flag) of a transformationunit is 0, and a transformation unit 1344 having a size of N×N is set ifa TU size flag is 1.

When the partition type is set to be asymmetrical, i.e., the partitiontype 1332, 1334, 1336, or 1338, a transformation unit 1352 having a sizeof 2N×2N is set if a TU size flag is 0, and a transformation unit 1354having a size of N/2×N/2 is set if a TU size flag is 1.

Split information (TU (Transformation Unit) size flag) of atransformation unit is a type of a transformation index. The size of thetransformation unit corresponding to the transformation index may bechanged according to a prediction unit type or partition type of thecoding unit.

For example, when the partition type is set to be symmetrical, i.e. thepartition type 2N×2N (1322), 2N×N (1324), N×2N (1326), or N×N (1328), atransformation unit 1342 having a size of 2N×2N is set if splitinformation (TU size flag) of a transformation unit is 0, and atransformation unit 1344 having a size of N×N is set if a TU size flagis 1.

When the partition type is set to be asymmetrical, i.e., the partitiontype 2N×nU (1332), 2N×nD (1334), nL×2N (1336), or nR×2N (1338), atransformation unit 1352 having a size of 2N×2N is set if a TU size flagis 0, and a transformation unit 1354 having a size of N/2×N/2 is set ifa TU size flag is 1.

Referring to FIG. 19, the TU size flag is a flag having a value or 0 or1, but the TU size flag is not limited to 1 bit, and a transformationunit may be hierarchically split having a tree structure while the TUsize flag increases from 0. Split information (TU size flag) of atransformation unit may be an example of a transformation index.

In this case, the size of a transformation unit that has been actuallyused may be expressed by using a TU size flag of a transformation unit,according to an embodiment of the present invention, together with amaximum size and minimum size of the transformation unit. According toan embodiment of the present invention, the video encoding apparatus 100is capable of encoding maximum transformation unit size information,minimum transformation unit size information, and a maximum TU sizeflag. A result of encoding the maximum transformation unit sizeinformation, the minimum transformation unit size information, and themaximum TU size flag may be inserted into an SPS. According to anembodiment of the present invention, the video decoding apparatus 200may decode video by using the maximum transformation unit sizeinformation, the minimum transformation unit size information, and themaximum TU size flag.

For example, (a) if the size of a current coding unit is 64×64 and amaximum transformation unit size is 32×32, (a-1) then the size of atransformation unit may be 32×32 when a TU size flag is 0, (a-2) may be16×16 when the TU size flag is 1, and (a-3) may be 8×8 when the TU sizeflag is 2.

As another example, (b) if the size of the current coding unit is 32×32and a minimum transformation unit size is 32×32, (b-1) then the size ofthe transformation unit may be 32×32 when the TU size flag is 0. Here,the TU size flag cannot be set to a value other than 0, since the sizeof the transformation unit cannot be less than 32×32.

As another example, (c) if the size of the current coding unit is 64×64and a maximum TU size flag is 1, then the TU size flag may be 0 or 1.Here, the TU size flag cannot be set to a value other than 0 or 1.

Thus, if it is defined that the maximum TU size flag is‘MaxTransformSizeIndex’, a minimum transformation unit size is‘MinTransformSize’, and a transformation unit size is ‘RootTuSize’ whenthe TU size flag is 0, then a current minimum transformation unit size‘CurrMinTuSize’ that can be determined in a current coding unit, may bedefined by Equation (1):CurrMinTuSize=max(MinTransformSize,RootTuSize/(2^MaxTransformSizeIndex))  (1)

Compared to the current minimum transformation unit size ‘CurrMinTuSize’that can be determined in the current coding unit, a transformation unitsize ‘RootTuSize’ when the TU size flag is 0 may denote a maximumtransformation unit size that can be selected in the system. In Equation(1), ‘RootTuSize/(2^MaxTransformSizeIndex)’ denotes a transformationunit size when the transformation unit size ‘RootTuSize’, when the TUsize flag is 0, is split a number of times corresponding to the maximumTU size flag, and ‘MinTransformSize’ denotes a minimum transformationsize. Thus, a smaller value from among‘RootTuSize/(2^MaxTransformSizeIndex)’ and ‘MinTransformSize’ may be thecurrent minimum transformation unit size ‘CurrMinTuSize’ that can bedetermined in the current coding unit.

According to an embodiment of the present invention, the maximumtransformation unit size RootTuSize may vary according to the type of aprediction mode.

For example, if a current prediction mode is an inter mode, then‘RootTuSize’ may be determined by using Equation (2) below. In Equation(2), ‘MaxTransformSize’ denotes a maximum transformation unit size, and‘PUSize’ denotes a current prediction unit size.RootTuSize=min(MaxTransformSize,PUSize)  (2)

That is, if the current prediction mode is the inter mode, thetransformation unit size ‘RootTuSize’, when the TU size flag is 0, maybe a smaller value from among the maximum transformation unit size andthe current prediction unit size.

If a prediction mode of a current partition unit is an intra mode,‘RootTuSize’ may be determined by using Equation (3) below. In Equation(3), ‘PartitionSize’ denotes the size of the current partition unit.RootTuSize=min(MaxTransformSize,PartitionSize)  (3)

That is, if the current prediction mode is the intra mode, thetransformation unit size ‘RootTuSize’ when the TU size flag is 0 may bea smaller value from among the maximum transformation unit size and thesize of the current partition unit.

However, the current maximum transformation unit size ‘RootTuSize’ thatvaries according to the type of prediction mode in a partition unit isjust an example and the present invention is not limited thereto.

The embodiments of the present invention may be written as computerprograms and may be implemented in general-use digital computers thatexecute the programs using a computer readable recording medium.Examples of the computer readable recording medium include magneticstorage media (e.g., ROM, floppy disks, hard disks, etc.) and opticalrecording media (e.g., CD-ROMs, or DVDs). The computer readablerecording medium may also be distributed over network coupled computersystems so that the computer readable code is stored and executed in adistributed fashion.

While the present invention has been particularly shown and describedwith reference to exemplary embodiments thereof, it will be understoodby those of ordinary skill in the art that various changes in form anddetails may be made therein without departing from the spirit and scopeof the present invention as defined by the following claims.

What is claimed is:
 1. A method for decoding a video, the methodcomprising: obtaining a coded block flag indicating whether atransformation block includes at least one non-zero significanttransformation coefficient; when the coded block flag indicates thetransformation block includes at least one non-zero significanttransformation coefficient, obtaining an x coordinate prefix bitstringof a last significant coefficient in the transformation block byperforming context-based-arithmetic decoding on a bitstream, obtaining ay coordinate prefix bitstring of the last significant coefficient byperforming context-based-arithmetic decoding on the bitstream, obtainingan x coordinate suffix bitstring of the last significant coefficient byperforming bypass mode decoding on the bitstream, and obtaining a ycoordinate suffix bitstring of the last significant coefficient byperforming bypass mode decoding on the bitstream; obtaining aninverse-binarized x coordinate prefix and an inverse-binarized ycoordinate prefix by performing inverse binarization according to afirst binarization method on the x coordinate prefix bitstring and the ycoordinate prefix bitstring; obtaining an inverse-binarized x coordinatesuffix and an inverse-binarized y coordinate suffix by performinginverse binarization according to a second binarization method on the xcoordinate suffix bitstring and the y coordinate suffix bitstring;reconstructing an x coordinate of the last significant coefficient basedon the inverse-binarized x coordinate prefix and the inverse-binarized xcoordinate suffix; and reconstructing a y coordinate of the lastsignificant coefficient based on the inverse-binarized y coordinateprefix and the inverse-binarized y coordinate suffix; obtaining, basedon the x coordinate and the y coordinate of the last significantcoefficient, significant maps of subsets having a predetermined sizewhich are split from the transformation block in a reverse index orderof the subsets; and reconstructing transformation coefficients in thetransformation block based on the significant maps, wherein, after the xcoordinate prefix bitstring and the y coordinate prefix bitstring areobtained via the context-based-arithmetic decoding, the x coordinatesuffix bitstring and the y coordinate suffix bitstring are obtained viathe bypass mode decoding, and wherein the transformation block is splitfrom coding unit based on a transformation block split information. 2.The method of claim 1, wherein the x coordinate indicates x-th column(where x is an integer equal to or greater than 0) in the transformationblock, and the y coordinate indicates y-th row (where y is an integerequal or greater than 0) in the transformation block.
 3. The method ofclaim 1, further comprising: reconstructing residuals in thetransformation block by performing inverse-quantization andinverse-transformation on the reconstructed transformation coefficients.4. The method of claim 1, wherein the last significant coefficient is anon-zero transformation coefficient scanned last in a scanning orderamong transformation coefficients in the transformation block.
 5. Themethod of claim 1, wherein an origin of the x coordinate and the ycoordinate of the last significant coefficient is an upper-leftmostpoint of the transformation block.